The present invention relates to a method for positioning a photosensitized substrate for printing a pattern on the mask onto the photosensitized substrate in an exposure apparatus used in a photolithographic process for fabricating semiconductor devices, image pick-up devices (such as charge-coupled devices), liquid crystal displays, thin film magnetic heads or the like. In particular, it relates to such a positioning method suitable for performing a coarse alignment (or prealignment) operation of a photo-sensitized substrate on a stage in an exposure apparatus.
In any of various exposure apparatuses used for fabrication of semiconductor devices, liquid crystal displays or the like including a projection exposure apparatus (such as a stepper) and a proximity printing exposure apparatus, in order to transfer a circuit pattern formed on a mask or reticle onto a photoresist film formed on a photosensitized substrate such as a wafer (or a glass plate, etc.) with high registration, it is required to establish alignment between the reticle and the wafer with precision.
For this purpose, there have been proposed various types of alignment sensor systems including: "laser step alignment (LSA) type" in which a wafer has an alignment mark formed thereon which comprises a linear array of dots, and a laser beam illuminates the alignment mark to produce diffracted or scattered beams, which are used to detect the position of the alignment mark (such as disclosed in U.S. Pat. No. 5,243,195); "field image alignment (FIA) type" in which an image-sensing device is used to take an image of an alignment mark which is illuminated by illumination light having a continuous spectrum of a wide wavelength range obtainable from a halogen lamp, and the picture data of the image is subjected to an image processing to measure the position of the alignment mark; and "laser interferometric alignment (LIA) type" in which a wafer has an alignment mark formed thereon which comprises a diffraction grating, two laser beams having different frequencies with a small difference between them illuminate the alignment mark from different directions to produce two diffracted beams interfering with each other, and the position of the alignment mark is determined from the phase of the interference. There have been also proposed various alignment techniques which may be categorized into three types including: "through-the-lens (TTL) type" in which the position of the wafer is measured through a projection optical system; "through-the-reticle (TTR) type" in which the relative position of a wafer with respect to a reticle is measured through both a projection optical system and the reticle; and "off-axis type" in which the position of the wafer is measured directly, or not through a projection optical system.
By using any of these alignment sensor systems to detect respective positions of two points on a wafer which is placed on a wafer stage, the rotational position (or rotational angle) of the wafer may be determined in addition to the position of the wafer with respect to the translational displacement. Alignment sensor systems usable for determining the rotational angle of a wafer include LIA-type system using TTL-type technique, LSA-type system using TTL-type technique, FIA-type system using off-axis-type technique, and others.
The exposure apparatus is required not only to have the ability of establishing alignment with precision between a reticle and a wafer by using the detection results obtained from the alignment sensor, but also to quickly establish such alignment so as to keep high throughput (the number of wafers that can be processed per unit of time). Thus, it is needed to realize highly effective operations in all the steps performed in relation to the exposure apparatus, from a transfer operation of a wafer onto the wafer stage to an exposure operation. Here, we will describe with reference to FIG. 1 the operations in the wafer loading step preceding the final wafer alignment step, as performed in a typical prior art exposure apparatus.
FIG. 1 shows a part of a wafer stage and the associated elements for a typical prior art exposure apparatus, for illustrating a wafer transfer mechanism. In FIG. 1, a lift device 19 is mounted on an X-stage 11 through a linear actuator 20. A substrate or wafer 6 has been loaded onto the lift device 19 from a wafer transfer unit (not shown). The lift device 19 has three support pins (of which only two support pins 19a and 19b are shown in FIG. 1) extending vertically through openings formed in a material support 9, a .theta.-rotation correction mechanism 8 and a wafer holder 7. The linear actuator 20 acts on the lift device 19 to lower/raise the three support pins, so as to lift down/up the wafer 6 for placing it onto and removing it from the wafer holder 7. Each support pin has at its tip end a hole selectively communicable with a vacuum source, which sucks the bottom surface of the wafer 6 to hold it, so as to prevent the wafer 6 from displacing horizontally upon vertical movements of the lift device 19.
Conventionally, a contact-prealignment process is used in which the peripheral edge of the wafer 6 is pressed against and engaged with a plurality of pins after the lift device 19 is lowered to place the wafer 6 onto the wafer holder 7. This prealignment process establish coarse alignment of the wafer 6 with respect to the rotational and translational offsets before the wafer 6 is held by vacuum suction to the wafer holder 7.
After the wafer 6 is held by vacuum suction and fixed to the wafer holder 7 in this manner, an alignment sensor system of a suitable type, such as the LSA type or the FIA type, is used to detect the alignment marks (search marks) formed on the wafer 6 at positions diametrically opposite to each other and produce detection signals. A movable mirror 13 mounted on the material support 9 and the associated laser interferometer disposed outside of the material support 9 together serve to measure the coordinates of the position of the material support 9. By determining such coordinates when the detection signals is at a peak, the translational errors and the rotational error of the wafer in terms of the wafer stage coordinate system are determined. The .theta.-rotation correction mechanism (or .theta.-table) is driven to vanish the rotational error of the wafer 6, so that the alignment in the rotational direction between the reticle and the wafer 6 (search alignment) is performed.
In this prior art technique, the .theta.-rotation correction mechanism 8 for rotating the wafer is disposed between material support 9 and the wafer 6, while the wafer stage coordinate system is defined with reference to the material support 9. Thus, there have been many problems including unwanted horizontal displacements of the wafer 6 which may occur due to insufficient suction for the vacuum-holding of the wafer by the wafer holder 7, a poor rigidity of the wafer stage due to the provision of various complicated mechanisms on the material support 9, as well as a poor controllability of the wafer stage due to the heavy weight thereof imposed by such complicated mechanisms. These problems could not be solved by disposing a .theta.-rotation correction mechanism under the material support 9 because the incident angle of the laser beam from the laser interferometer into the movable mirror 13 fixedly mounted on the material support 9 would change when the rotation angle of the wafer 6 is adjusted by driving the .theta.-rotation correction mechanism, so that the range of rotation angle of the .theta.-rotation correction mechanism would be limited, resulting in a disadvantage that errors in prealignment could not be corrected unless they are sufficiently small.
Furthermore, in this prior art exposure apparatus, the translational errors and the rotational error of the wafer 6 is detected by measuring the positions of two alignment marks (search marks) formed on the wafer 6 and distant from each other by means of a single alignment sensor system of the LSA or the FIA type, after the wafer 6 is held on the wafer holder 7. However, in order to detect two distant alignment marks by means of a single alignment sensor system, the wafer 6 has to be moved so as to position the alignment marks sequentially into the detection area of the alignment sensor system, and this operation has to be repeated for each of the wafers in one lot, resulting in low throughput of the exposure process. This problem could not be conveniently solved by providing two alignment sensor systems for simultaneous detection of the two alignment marks, because the arrangement of the two alignment sensor systems on the exposure apparatus imposes a limitation on the arrangement of the two alignment marks on the wafer 6, so that it would be difficult for such exposure apparatus to accommodate wafers of different sizes, for example.
In this relation, the last problem could not be conveniently solved by providing an adjustor mechanism for adjusting the distance between the two alignment sensor systems because such adjustor mechanism has to be inherently complicated, is difficult to dispose in the space around the wafer stage where various sensors and other components are closely disposed, and tends to increase the manufacture costs.
Also, various contact-prealignment mechanisms have been used to establish coarse alignment after the wafer has been placed on the wafer holder 7. However, there is an disadvantage that high throughput can not be obtained when the prealignment operation has to be performed after the wafer has been placed on the wafer holder. Nevertheless, it is desirable that any exposure apparatus having a newly proposed prealignment mechanism which may provide high throughput, may be capable of providing the matching with another, existing exposure apparatus having a conventional contact-prealignment mechanism.
The orientation of a wafer can be corrected to meet a predefined reference by performing the prealignment process, in which a noncontact-type prealignment mechanism is used to measure the outer contour of the peripheral edge of the wafer and the orientation of the wafer is so corrected as to coincide with the reference. However, this prealignment process still suffers from a problem: in the case that the alignment marks on the surface of a wafer, which have been formed through a previous lithographic process, offset (in a rotational direction) away from the desired positions that are predefined relative to the geometrical features (such as an orientation flat and a notch) defined by the configuration of the peripheral edge of the wafer, the fine alignment process following the prealignment process may not be performed efficiently, or it may even be impossible at all to perform the fine alignment process so the wafer has to be rejected as a failure.